Plants, unlike animals, do not have a circulatory system to transport substances over long distances. Yet, they need to move water from the roots to the highest parts of the stem and photosynthesised food from the leaves to all other plant parts, including the roots deep in the soil.

Movement also occurs over short distances, such as within a single cell, across cell membranes, and from cell to cell within a tissue.

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Substances transported in flowering plants include water, mineral nutrients, organic nutrients (like sugars), and plant growth regulators (hormones).

• Short-distance transport: Occurs within a cell, across membranes, or cell to cell within tissues, primarily by diffusion, cytoplasmic streaming, and active transport.
• Long-distance transport: Occurs over longer distances through the plant's vascular system (xylem and phloem). This process is called translocation.

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Direction of transport:

• In rooted plants, water and minerals transport in xylem is typically unidirectional, moving upwards from roots to stems and leaves.
• Organic and mineral nutrients can be transported in multiple directions. Organic compounds made in leaves are moved to storage organs or areas of high demand (sinks). Nutrients can also be withdrawn from aging plant parts (senescing) and moved to younger, growing regions.
• Plant growth regulators and chemical signals are transported in small amounts, sometimes in a strict unidirectional (polar) manner from their synthesis site.

Overall, there is a complex, yet organized, movement of substances in various directions within a flowering plant, with different organs acting as both sources and sinks depending on their needs.

Means Of Transport

Substances move within plants using various mechanisms, depending on the distance and the nature of the substance.

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Diffusion

Diffusion is a passive process, meaning it does not require energy expenditure by the cell. It is the movement of molecules from a region of higher concentration to a region of lower concentration, driven by the random motion of molecules and the concentration gradient.

Diffusion is a relatively slow process and is effective only for movement over short distances (within a cell, across membranes, or between adjacent cells).

It does not depend on a living system and occurs readily in gases and liquids. While diffusion of solids is possible, diffusion *in* solids is more common (e.g., a gas diffusing through a solid). In plants, diffusion is the only means for gaseous movement within the plant body.

The rate of diffusion is influenced by:

• The concentration gradient.
• Permeability of any membrane involved.
• Temperature.
• Pressure.

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Facilitated Diffusion

This is also a passive process that does not require ATP energy, occurring down a concentration gradient. However, it involves the assistance of membrane proteins to move substances across the membrane.

Substances that are hydrophilic (water-soluble) and cannot easily pass through the lipid bilayer of the cell membrane require facilitation for their movement.

Special membrane proteins, called transport proteins or carriers, provide specific sites for these molecules to cross the membrane (Figure 11.1).

Characteristics of Facilitated Diffusion:

• Requires a concentration gradient (cannot move substances from low to high concentration without energy).
• Transport rate reaches a maximum (saturation) when all carrier proteins are being used.
• It is highly specific; transport proteins bind only to specific substances.
• It is sensitive to inhibitors that interact with the protein carriers.

Membrane proteins involved in facilitated diffusion can form channels that are always open or can be controlled (gated). Some are large pores called porins, found in outer membranes of plastids, mitochondria, and some bacteria, allowing even small proteins to pass. Water channels, made of proteins called aquaporins, are another example.

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Passive Symports And Antiports

Some carrier proteins facilitate the diffusion of two different types of molecules together across the membrane (Figure 11.2).

• Symport: Both molecules move across the membrane in the same direction.
• Antiport: The two molecules move in opposite directions across the membrane.

If a single molecule moves across the membrane independently of other molecules, the process is called uniport.

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Active Transport

Active transport is the movement of molecules across a membrane against a concentration gradient (from low concentration to high concentration) or against an electrochemical gradient. This process requires metabolic energy in the form of ATP.

Active transport is carried out by specific membrane proteins, often referred to as pumps, which utilize ATP energy to transport substances uphill.

Characteristics of Active Transport:

• Requires ATP energy.
• Moves substances against the concentration gradient (uphill transport).
• Carried out by specific membrane proteins (pumps).
• Transport rate reaches a maximum (saturation) when all protein transporters are engaged.
• Highly specific for the transported substance.
• Sensitive to inhibitors.

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Comparison Of Different Transport Processes

The table below summarizes the key differences between simple diffusion, facilitated diffusion, and active transport (Table 11.1).

Property	Simple Diffusion	Facilitated Transport	Active Transport
Requires special membrane proteins	No	Yes	Yes
Highly selective	No	Yes	Yes
Transport saturates	No	Yes	Yes
Uphill transport	No	No	Yes
Requires ATP energy	No	No	Yes

Both facilitated diffusion and active transport involve membrane proteins, making them highly selective and prone to saturation and inhibition. However, only active transport moves substances against a gradient and uses energy, while diffusion (simple or facilitated) always occurs down a gradient without energy input.

Plant-Water Relations

Water is fundamental for all life processes in plants, serving as a medium for dissolving and transporting substances. Protoplasm, the living content of a cell, is essentially water with dissolved and suspended molecules.

Water content varies in plants. Watermelon is over 92% water, while herbaceous plants are typically 10-15% dry matter. Even seemingly dry seeds contain water necessary for survival and respiration.

Terrestrial plants absorb large amounts of water daily, but most is lost to the atmosphere through transpiration (evaporation from leaves). A corn plant can absorb $\sim 3$ liters per day, and a mustard plant can absorb its own weight in water in about 5 hours.

This high water demand makes water a critical limiting factor for plant growth and productivity in many environments.

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Water Potential

Understanding water movement in plants requires the concept of water potential ($\textsf{Ψ}_\textsf{w}$), which represents the potential energy of water molecules.

Water molecules are in constant random motion. The higher the concentration of water, the higher its kinetic energy and water potential. By convention, pure water at standard temperature and atmospheric pressure has a water potential of zero ($\textsf{Ψ}_\textsf{w} = 0$).

Water moves from a system with higher water potential to one with lower water potential (down the gradient of free energy). This movement is a type of diffusion.

Water potential is measured in units of pressure, such as pascals (Pa).

Water potential ($\textsf{Ψ}_\textsf{w}$) is determined by two main components:

1. Solute Potential ($\textsf{Ψ}_\textsf{s}$): Adding a solute to pure water reduces the concentration of free water molecules, lowering its water potential. Solute potential is the magnitude of this reduction and is always negative. A higher solute concentration leads to a more negative $\textsf{Ψ}_\textsf{s}$.
2. Pressure Potential ($\textsf{Ψ}_\textsf{p}$): The effect of physical pressure on water potential. Applying positive pressure (e.g., turgor pressure in a plant cell) increases water potential ($\textsf{Ψ}_\textsf{p}$ is usually positive). Negative pressure (tension), such as the pull in the xylem during transpiration, reduces water potential ($\textsf{Ψ}_\textsf{p}$ is negative).

The relationship is expressed as: $\textsf{Ψ}_\textsf{w} = \textsf{Ψ}_\textsf{s} + \textsf{Ψ}_\textsf{p}$.

For pure water at atmospheric pressure, $\textsf{Ψ}_\textsf{s} = 0$, so $\textsf{Ψ}_\textsf{w} = \textsf{Ψ}_\textsf{p}$. At atmospheric pressure, $\textsf{Ψ}_\textsf{w} = 0$ for pure water.

For a solution at atmospheric pressure ($\textsf{Ψ}_\textsf{p} = 0$), $\textsf{Ψ}_\textsf{w} = \textsf{Ψ}_\textsf{s}$.

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Osmosis

Osmosis is specifically the diffusion of water across a selectively or differentially permeable membrane (a membrane that allows some substances, like water, to pass freely while restricting others, like solutes). Osmosis occurs in response to differences in water potential.

Net movement of water occurs from a region of higher water potential (lower solute concentration) to a region of lower water potential (higher solute concentration) until equilibrium is reached.

The rate and direction of osmosis depend on both the concentration gradient (leading to solute potential) and the pressure gradient.

In plant cells, the cell wall is freely permeable, but the cell membrane and the vacuolar membrane (tonoplast) are selectively permeable and control water movement into or out of the protoplast.

Osmotic pressure is the pressure required to prevent the net diffusion of water into a solution across a semi-permeable membrane. Numerically, osmotic pressure is equivalent to the osmotic potential ($\textsf{Ψ}_\textsf{s}$), but its sign is positive (osmotic potential is negative). Higher solute concentration leads to higher osmotic pressure.

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Plasmolysis

Plasmolysis is the process where the protoplast (cell membrane and cytoplasm) of a plant cell shrinks away from the cell wall when the cell is placed in a hypertonic solution (a solution with a higher solute concentration, and thus lower water potential, than the cell's cytoplasm).

In a hypertonic solution, water moves out of the cell by osmosis, initially from the cytoplasm and then from the vacuole. As water leaves, the protoplast shrinks and pulls away from the rigid cell wall (Figure 11.5).

The space between the cell wall and the shrunken protoplast in a plasmolysed cell is typically filled with the external hypertonic solution.

In an isotonic solution (a solution with the same osmotic pressure as the cytoplasm, thus equal water potential), there is no net movement of water. The cell is said to be flaccid when water entering and leaving the cell are in equilibrium, resulting in no turgor pressure.

When a plasmolysed cell is placed in a hypotonic solution (a solution with lower solute concentration and higher water potential than the cytoplasm), water diffuses back into the cell by osmosis. This inflow of water causes the protoplast to swell and press against the cell wall, building up turgor pressure ($\textsf{Ψ}_\textsf{p}$). The rigid cell wall resists this pressure, preventing the cell from bursting. Turgor pressure is responsible for maintaining cell rigidity and contributes to plant support and growth.

A flaccid cell has zero turgor pressure ($\textsf{Ψ}_\textsf{p} = 0$). In a turgid cell, turgor pressure is maximal and equal to the osmotic potential, so $\textsf{Ψ}_\textsf{w} = \textsf{Ψ}_\textsf{s} + \textsf{Ψ}_\textsf{p} = 0$.

Organisms other than plants that possess a cell wall include bacteria, fungi, and algae.

Plasmolysis is usually reversible if the cell is transferred to a hypotonic solution before damage is permanent.

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Imbibition

Imbibition is a specialized type of diffusion where certain solid materials (colloids) absorb water or other liquids, causing them to swell and significantly increase in volume.

Examples: Absorption of water by dry seeds, dry wood (historically used to split rocks).

Significance: Imbibition pressure generated by swelling seeds is crucial for seedlings to break through the soil surface during germination and become established.

Imbibition, like other forms of diffusion, occurs down a water potential gradient. Dry seeds have very low (highly negative) water potential, driving water absorption from the surrounding environment. Affinity between the absorbent material (adsorbant) and the liquid is also essential for imbibition to occur.

Long Distance Transport Of Water

Transport of water and other substances over long distances in plants cannot rely solely on diffusion, as diffusion is too slow for such distances (e.g., movement across a 1m distance by diffusion alone would take many years).

In large plants, special long-distance transport systems are necessary for rapid movement of substances over significant distances, especially between absorption/production sites and storage/utilization sites.

Water, minerals, and food are transported over long distances by a system called mass flow or bulk flow. This is the movement of substances in bulk from one point to another due to a pressure difference between the two points.

In mass flow, all substances within the moving fluid (whether dissolved or suspended) are carried along at the same rate, unlike diffusion where each substance moves independently based on its own gradient.

Bulk flow can be driven by: a positive hydrostatic pressure gradient (like water flowing from a tap) or a negative hydrostatic pressure gradient (like sucking liquid through a straw).

The bulk movement of substances through the plant's vascular tissues (xylem and phloem) is called translocation.

• Xylem: Primarily translocates water and mineral salts absorbed from the roots to the aerial parts. Also transports some organic nitrogen and hormones. Movement is generally unidirectional upwards.
• Phloem: Translocates a variety of organic and inorganic solutes, mainly sugars (food) produced in the leaves, to other parts of the plant. Movement can be bi-directional (upwards or downwards).

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How Do Plants Absorb Water?

Most water absorbed by plants is taken up by the roots from the soil. This function is primarily carried out by millions of microscopic root hairs located near the root tips.

Root hairs are thin-walled extensions of root epidermal cells that vastly increase the surface area for absorption of water and minerals. Water absorption by root hairs is a passive process driven by diffusion.

Once water is absorbed by root hairs, it moves deeper into the root through two main pathways (Figure 11.6):

1. Apoplast pathway: Movement of water occurs exclusively through the non-living components – the intercellular spaces and the cell walls of the cortex cells. This movement does not involve crossing any cell membrane (except possibly initially into the root hair). It is a mass flow driven by tension created by transpiration. The apoplast is continuous throughout the plant, except at the Casparian strips of the endodermis.
2. Symplast pathway: Movement of water occurs through the living parts of the root – the cytoplasm of the cortex cells. Water enters the cells by crossing the cell membrane and moves from cell to cell through the interconnected cytoplasm via plasmodesmata (cytoplasmic strands connecting adjacent protoplasts). This movement is relatively slower as it involves crossing membranes and is sometimes aided by cytoplasmic streaming.

In the root cortex, water primarily moves via the apoplast due to the loosely packed cells offering little resistance. However, the endodermis, the innermost layer of the cortex, has a band of suberin (a water-impermeable material) called the Casparian strip in its radial and tangential walls. The Casparian strip blocks the apoplastic movement of water.

At the endodermis, water is forced to enter the endodermal cells by crossing the cell membrane (symplastic pathway) to reach the vascular cylinder (xylem). This is a control point where the plant regulates water and solute movement into the xylem.

Once water is inside the xylem vessels and tracheids (which are non-living conduits), it is again part of the apoplast and can move freely up the plant.

Mycorrhiza: Many plants form a symbiotic association with fungi called mycorrhiza. Fungal hyphae associate with roots, either surrounding them or penetrating cells. The hyphae extend into the soil, increasing the surface area for absorption of water and minerals, particularly phosphorus, beyond what root hairs alone can reach. In return, the plant provides the fungus with sugars and nitrogen compounds. Some plants, like *Pinus*, depend obligately on mycorrhizal association for germination and establishment.

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Water Movement Up A Plant

After water enters the xylem, it needs to be transported upwards to the rest of the plant. This movement occurs against gravity. Two main mechanisms are proposed for this ascent of water:

1. Root Pressure:

Root pressure is a positive hydrostatic pressure that develops in the xylem sap of roots due to the active uptake of ions from the soil. As ions are pumped into root vascular tissues, water follows by osmosis, increasing pressure within the xylem.

This pressure can push water up to a small height in the stem.

Evidence of root pressure: Guttation, the loss of water in liquid form from specialized pores (hydathodes) near leaf tips, often observed in small herbaceous plants during the night or early morning when transpiration is low. Another evidence is the exudation of sap from a cut stem base (bleeding).

Role of root pressure: Provides a modest push, helps re-establish continuous water columns in the xylem (which might break under tension), but is insufficient to transport water to the top of tall trees. Most water transport is due to transpiration pull.

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Transpiration Pull

This is the most widely accepted model for water transport in tall plants, known as the Cohesion-Tension-Transpiration Pull model. It proposes that water is 'pulled' up the plant, driven by transpiration (evaporative water loss) from the leaves.

Water loss through stomata in leaves creates a negative pressure or tension in the xylem vessels.

This tension is transmitted down the continuous column of water in the xylem from the leaves to the roots.

The properties of water that make this possible are:

• Cohesion: Strong mutual attraction between water molecules due to hydrogen bonding.
• Adhesion: Attraction of water molecules to the polar surfaces of the xylem elements (tracheids and vessels).
• Surface tension: Water molecules are more attracted to each other in the liquid phase than to the gas phase.

These properties provide water with high tensile strength (ability to resist pulling) and high capillarity (ability to rise in narrow tubes, aided by the small diameter of xylem elements).

Mechanism:

• Water evaporates from the thin film on the surface of mesophyll cells into the intercellular spaces of the leaf.
• From the intercellular spaces, water vapour diffuses out of the leaf through the stomata into the drier outside air (down a water vapour concentration gradient).
• As water evaporates from the leaf cells, it creates a negative potential (tension) that pulls water from adjacent cells.
• This pull is transmitted backwards through the continuous water column in the xylem, all the way to the roots.
• The cohesive and adhesive properties of water maintain the integrity of the water column against the pulling tension.

This transpiration-driven pull is strong enough to raise water columns over 130 meters high.

Transpiration

Transpiration is the process of water loss from plant parts in the form of water vapour. It occurs primarily through small pores on the leaf surface called stomata.

Stomata are also the main sites for the exchange of oxygen and carbon dioxide during photosynthesis and respiration.

Stomatal opening and closing are regulated by changes in the turgidity of the surrounding guard cells (Figure 11.8).

• Guard cells have thickened inner walls (towards the pore) and thinner outer walls.
• When guard cells become turgid due to water uptake, the thin outer walls bulge, forcing the thicker inner walls into a crescent shape and opening the stoma. The radial arrangement of cellulose microfibrils in guard cell walls aids this opening.
• When guard cells lose turgor (become flaccid), the inner walls regain their original shape, and the stoma closes.

Distribution of stomata: Generally, dicotyledonous leaves (dorsiventral) have more stomata on the lower surface, while monocotyledonous leaves (isobilateral) have roughly equal numbers on both surfaces.

Factors affecting transpiration rate:

• External factors: Temperature, light intensity, humidity of the air, wind speed.
• Plant factors: Number and distribution of stomata, percentage of open stomata, water status of the plant, canopy structure.

The ascent of sap (xylem sap movement) is driven by the physical properties of water (cohesion, adhesion, surface tension) as described in the transpiration pull model (Figure 11.9).

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Transpiration And Photosynthesis – A Compromise

Transpiration is essential for plants, but it also involves significant water loss, which can limit photosynthesis, especially in water-stressed conditions.

Benefits of transpiration:

• Generates the transpiration pull needed for absorbing and transporting water and minerals upwards.
• Supplies water for photosynthesis.
• Transports minerals from the soil to all plant parts.
• Provides cooling effect to leaf surfaces through evaporative cooling ($\sim 10-15^\circ\textsf{C}$).
• Helps maintain plant shape and structure by keeping cells turgid.

The need for water for photosynthesis creates a conflict with the need to minimize water loss through transpiration. Plants have evolved strategies to manage this, such as the development of the C4 photosynthetic pathway.

C4 plants are more efficient at fixing CO$_2$ than C3 plants and lose only about half as much water for the same amount of CO$_2$ fixed, representing a compromise between maximizing CO$_2$ uptake and minimizing water loss.

Rainforests maintain high humidity largely due to the massive amount of water cycling through plants via transpiration.

Uptake And Transport Of Mineral Nutrients

Plants obtain carbon and most oxygen from atmospheric CO$_2$. Other essential nutrients, primarily mineral elements, are absorbed from water and soil.

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Uptake Of Mineral Ions

Uptake of mineral ions by roots is more complex than water absorption, requiring both passive and active transport mechanisms.

Reasons why passive absorption is often insufficient for minerals:

1. Mineral elements exist as charged particles (ions), which cannot easily cross the nonpolar lipid bilayer of cell membranes.
2. The concentration of minerals in the soil is usually lower than that inside the root cells (creating a concentration gradient against which transport is needed).

Therefore, most minerals enter root epidermal cells via active absorption into the cytoplasm. This process requires ATP energy and is mediated by specific transport proteins (pumps) in the root hair cell membranes.

Some mineral ions may also enter passively if a favourable gradient exists.

The active uptake of ions contributes to lowering the water potential in root cells, which in turn facilitates water uptake by osmosis.

Control points: The endodermal cells in the root play a significant role in regulating the amount and type of ions reaching the xylem. Endodermal cells have specific transport proteins in their membranes that selectively allow solutes to cross. The Casparian strip forces all ions to pass through the endodermal cell cytoplasm (symplast), giving the plant control over ion transport into the vascular cylinder. Endodermal transport proteins actively move ions in one direction only (towards the xylem).

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Translocation Of Mineral Ions

Once mineral ions reach the xylem (via active or passive uptake into the symplast and passage through the endodermis), they are transported upwards to various plant parts primarily through the transpiration stream along with water.

Sinks for mineral elements are typically actively growing regions:

• Apical and lateral meristems
• Young leaves
• Developing flowers, fruits, and seeds
• Storage organs

Unloading of mineral ions at the sink tissues occurs via diffusion or active uptake by the cells that require them.

Remobilisation of minerals: Mineral nutrients can be withdrawn from older, senescing (aging) plant parts and transported to younger, growing regions. Elements that are readily mobile include phosphorus, sulphur, nitrogen, and potassium. Elements that are structural components (like calcium) are generally not remobilised from aging tissues.

Transport form: While some nitrogen is transported as inorganic ions in the xylem sap, a significant portion, along with small amounts of phosphorus and sulphur, is transported in organic forms (e.g., amino acids and related compounds).

There is also some exchange of materials between xylem and phloem. This suggests that the traditional view that xylem transports only inorganic substances and phloem only organic substances is not entirely accurate, as some inorganic substances are transported in organic forms and some exchange occurs.

Phloem Transport: Flow From Source To Sink

Phloem is the vascular tissue responsible for translocating food, primarily the sugar sucrose, from a source (where it is produced or stored) to a sink (where it is needed or stored).

Typically, the source is a photosynthetic leaf, and the sink is a root, fruit, seed, or growing bud. However, the source-sink relationship is not fixed and can be reversed depending on the plant's physiological needs and the season.

Example: In early spring, sugars stored in roots can be mobilized and transported upwards to developing buds (which act as sinks requiring energy for growth), while the roots become the source.

Direction of movement in phloem: Due to the variable source-sink relationship, transport in the phloem can be bi-directional (upwards or downwards), unlike the unidirectional upward movement in xylem.

Phloem sap: The fluid transported in phloem consists mainly of water and sucrose, but also contains other sugars, hormones, and amino acids.

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The Pressure Flow Or Mass Flow Hypothesis

This is the most widely accepted model explaining the mechanism of sugar translocation in phloem (Figure 11.10).

Steps involved:

1. Loading at the Source:
   • Sugars produced by photosynthesis (initially glucose) are converted into sucrose.
   • Sucrose is actively transported into the companion cells and then into the phloem sieve tube elements (cells) at the source. This process is called phloem loading.
   • Active loading of sugars creates a high concentration of solutes in the sieve tube elements at the source.
2. Creation of Pressure Gradient:
   • The high solute concentration in the sieve tube elements at the source makes their water potential low (hypertonic condition).
   • Water from the adjacent xylem vessels moves into the sieve tube elements by osmosis, increasing the hydrostatic pressure (turgor pressure) in the sieve tubes at the source.
3. Mass Flow:
   • The region of high pressure at the source drives the bulk movement of phloem sap (containing sugars and water) through the sieve tubes towards regions of lower pressure (the sink).
   • This is a mass flow phenomenon; sap flows from high pressure to low pressure.
4. Unloading at the Sink:
   • At the sink, sucrose is actively transported out of the sieve tube elements into the sink cells (where it is used for metabolism, growth, or storage, e.g., converted to starch or cellulose). This process is called phloem unloading.
   • Removal of sugars decreases the solute concentration in the sieve tube elements at the sink, increasing their water potential.
5. Water Return:
   • As the osmotic pressure decreases in the sieve tubes at the sink, water moves out by osmosis, typically returning to the xylem.

Thus, phloem loading at the source creates a pressure gradient that drives mass flow, while phloem unloading at the sink reduces the pressure, completing the transport cycle.

Girdling experiment: Removing a ring of bark including phloem from a tree trunk demonstrates that phloem is the tissue responsible for food translocation. The bark above the ring swells due to the accumulation of sugars that cannot be transported downwards to the roots, confirming downward transport of food in the phloem.

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